Solutions Using RDP

*Semantic actions are under-lined
1) Assignment Statement
A1) A -> id = E { gen_instr (POPM, get_address(id)) }
A2) E -> T E’
A3) E’ -> + T { gen_intsr (ADD, nil) } E’
A4) E’ -> ε
A5) T -> F T’
A6) T’ -> *F { gen_instr (MUL, nil) } T’
A7) T’ -> ε
A8) F -> id { gen_instr (PUSHM, get_address(id) )
Procedure A ()
{
If token = id then
{
save = token;
lexer();
If token = “=” then
{
lexer();
E();
get_instr (POPM, get_address (save) );
}
else error_message ( “= expected” );
}
else error_massage ( “ id expected” );
}
Procedure E ():
{
T ();
E’();
}
2
Procedure E’();

{
If token = “+” the
n

{
lexer();
T();

gen_instr (ADD, nil);
E’();

}
}
;
Procedure T(); {
F();
T’(); }
Proc
edure T’()
{
If token = “*” then

{
lexer();
F();
gen_instr(MUL, nil);
T’();

}
}
Procedure F(); {
If token = id then

{
gen_instr(PUSHM, get_address (token));
lexer();

}
else error_message(“id expected”); };
3
Procedure gen_instr(op, oprnd)
/* instr_address shows the current insturction address is global / { Instr_table [instr_address].address = inst_address; Instr_table [instr_address].op = op; Instr_table [instr_address].oprnd = oprnd; Instr_address++; }; Example: x = a + bc
(addresses a = 5001 , b=5002, c=5003 and x = 5004)
INSTR_TABLE
address Op Oprnd
1 PUSHM 5001
2 PUSHM 5002
3 PUSHM 5003
4 MUL nil
5 ADD nil
6 POPM 5004
Print from INSTR_TABLE ignoring “nil”
4

2. While Statement
   W1) W -> while ( C ) S
   W2) C - > E R E
   W3) R -> == | > | <
   Procedure while_statement();
   {
   If token = “while” then
   {
   addr = instr_address;
   gen_instr(“LABEL”, nil);
   lexer();
   If token = “(“ then
   {
   lexer();
   C ( );
   If token = “)” then
   {
   lexer();
   S();
   gen_instr(JUMP, addr);
   back_patch (instr_address);
   lexer();
   };
   else error_message (“ ) expected”);
   else error_message (“( expected”);
   }
   else error_message (“while expected”);
   };
   5
   Procedure C ()
   {
   E();
   If token in R then
   {
   op = token;
   lexer();
   E();
   case op of
   < : gen_instr (LES, nil);
   push_jumpstack (instr_address); /* another stack need */
   gen_instr (JUMPZ, nil);

:
== :
}
else error_message (“ R token expected”);
}
Procedure back_patch (jump_addr)
{
addr = pop_jumpstack();
Instr_table[addr].oprn = jump_addr;
}
6
Example: while ( i < max) i = i + 1;
with addresses I =5000, max = 5001

1. LABEL nil
2. PUSHM 5000
3. PUSHM 5001
4. LES nil
5. JUMPZ 11 /* back patch */
6. PUSHM 5000
7. PUSHM 5001
8. ADD nil
9. POPM 5000
10. JUMP 1
11. …..
    7
12. if statement
    I -> if ( C ) S fi
    Procedure I ();
    {
    If token =”if” then
    {
    addr = instr_address();
    lexer();
    If token =”(“ then
    {
    lexer();
    C( );
    If token = “)” then
    {
    lexer();
    S( );
    back_patch(instr_address);
    If token = “fi”
    lexer();
    else error_messgage (“fi expected “);
    }
    else error_message (“) expected “);
    };
    else error_message (“( expected”);
    }
    else error_message (“if expected”);
    };
    8
    Example: if (a < b) a = c; fi
    With addresses a = 5000, b = 5001, c = 5002
13. PUSHM 5000
14. PUSHM 5001
15. LES nil
16. JUMPZ 7
17. PUSHM 5002
18. POPM 5000
19. LABEL
    NOTE:

• You need work on , and statement
• DO NOT create your own instructions